What is GHSA-q2h6-ghwm-5qm8?

GHSA-q2h6-ghwm-5qm8 is a security advisory published by GitHub Security Advisories with Medium severity. MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings

Which CVE IDs does GHSA-q2h6-ghwm-5qm8 cover?

GHSA-q2h6-ghwm-5qm8 covers the following CVE IDs: CVE-2026-48516. Each CVE has its own detail page on EchelonGraph Pulse with the synthesized EG score.

Which products are affected by GHSA-q2h6-ghwm-5qm8?

GHSA-q2h6-ghwm-5qm8 affects 2 products, including: nuget/MessagePack:\u003c 2.5.301; nuget/MessagePack:\u003e= 3.0, \u003c 3.1.7.

GHSA-q2h6-ghwm-5qm8Medium

MessagePack-CSharp: InterfaceLookupFormatter bypasses collision-resistant comparer settings

Vendor

GitHub Security Advisories

Published

June 25, 2026

Last Modified

June 25, 2026

🔗 CVE IDs covered (1)

CVE-2026-48516 →

📋 Description

Summary

InterfaceLookupFormatter<TKey,TElement> constructs an internal Dictionary<TKey, IGrouping<TKey,TElement>> with the default equality comparer instead of the security-aware comparer supplied by options.Security.GetEqualityComparer<TKey>().

Other hash-based collection formatters use the security-aware comparer when MessagePackSecurity.UntrustedData is configured. This formatter omission allows hash-collision CPU denial of service against ILookup<TKey,TElement> even when the application has opted into the untrusted-data security posture.

Impact

Applications are affected when they deserialize untrusted payloads into schemas containing ILookup<TKey,TElement> with a key type for which attacker-controlled hash collisions are feasible.

Under the default comparer, many colliding keys can degrade dictionary insertion from amortized constant time to quadratic behavior. A payload of colliding keys can consume CPU for a disproportionate amount of time. This bypasses the mitigation that developers intentionally enabled by using MessagePackSecurity.UntrustedData.

Affected components

Package: MessagePack
API: InterfaceLookupFormatter<TKey,TElement>.Create
Data type: ILookup<TKey,TElement>
Finding ID: MESSAGEPACKCSHARP-041

Patches

Fixes are prepared and will be released in coordinated patch versions.

Upgrade guidance:

Upgrade MessagePack to the patched version for your release line.
Upgrade companion MessagePack packages in the same dependency graph to the coordinated patched versions.

The fix should create the internal dictionary with options.Security.GetEqualityComparer<TKey>(), matching the sibling dictionary and lookup formatter behavior.

Workarounds

Patching is recommended.

Until a patched version is available, avoid exposing ILookup<TKey,TElement> in DTOs that deserialize untrusted data. Use collection shapes that are already protected by the security-aware comparer path, or validate and cap collection sizes at the transport boundary.